Directional antenna system



. Oct. 24, 1939. ALFORD 2,177,415

- DIRECTIONAL ANTENNA SYSTEM Filed Dec. 31, 1936 FIG. 1. 208 R62.

X3019 7 all 3/6 3/0 INVENTOR I REW Alf'flRD ATTORN EY Patented Oct. 24, 1939 UNITED STATES DIRECTIONAL ANTENNA SYSTEM Andrew Alford, New York, N. Y., assignor to Mackay Radio and Telegraph Company, New York, N. Y., a corporation of Delaware Application December 31, 1936, Serial No. 118,541

This invention relates to directive antenna systems and particularly to a method of and apparatus for obtaining unidirectional radiation at more than one frequency.

In order to obtain unidirectional radiation from an antenna it has been the practice to construct an antenna consisting of two parts, one acting as a reflector for the other. The forward part or the part located closer to the rem, ceiving station has been usually designated as the radiator while the back part or the part further away from the receiving station has been designated as a reflector. when the two parts, the radiator and the reflector, are spaced an odd number of quarter wave lengths from each other and are fed in phase quadrature the radiation which results from the combination of both parts when each part receives one-half the total power is unidirectional.

Since the distance between the reflector and the radiator must be equal to an odd number of quarter Wave lengths, the application of this kind of an antenna system is limited to just one particular frequency, for if this distance is not equal to an odd number of quarter wave lengths but is different from this number by even a small amount then the antenna system radiates in both directions, that is, from A toward B as well as from B toward A.v 30L.

antenna system comprising two antennae, which is adapted to produce unidirectional radiation at a plurality of frequencies.

A further object of my invention is to provide a unidirectional antenna system having two antennae which are spaced apart an arbitrary distance which is independent of the wavelengths transmitted.

A still further object of my invention is to pro- ,{o's vide an antenna system having two antennae which are spaced an arbitrary distance apart, and which is adapted to provide a-unidirectional transmission at two difierent frequencies.

In accordance with my invention it is, there- .5 fore, possible to obtain unidirectional radiation from an antenna system including two antennae, which are spaced an arbitrary distance apart, the spacing distance not being necessarily an odd number of quarter wave lengths of the frequency 503' or frequencies which it is desired to transmit.

The present invention makes it possible to build just one antenna system instead of the usual two for the day frequency and the intermediate frequency. And also makes it possible 55' to change the frequencies transmitted on t It is well known that It is an object of my invention to provide an 16 Claims. (01. 250-511 existing antennae, whenever required, without moving the towers or poles which support the antennae.

Broadly speaking, the objects of my invention are obtained by modifying the phase of reflection at the end of the antenna wires in such a way that the waves which reflect at the end of the wires and which give rise to back radiation,

' produce equal and opposite fields in the direction opposite to that of the receiving station. The desired modification of phase at any particular frequency may be made in either of the antennae, the radiator or the reflector, or in both of them, and may consist in either an advancement of phase or a retardation of phase. The phase of either or both antennae may be advanced or retarded, or the phase of one antenna may be advanced and that of the'other retarded. In any case the desired result of cancellation of the backward radiation may be attained.

The above mentioned and further objects and advantages of my invention, and the manner of attaining them, will be more fully explained in the following description and accompanying drawing.

In the drawing,

Fig. 1 shows an antenna embodying my invention, which is adapted for unidirectional use at two frequencies, and Figs, 2 to 4 illustrate modified types of antennae embodying my invention.

Referring more particularly to the drawing, in Fig. l is illustrated an antenna system including an antenna A, which acts as a radiator, and an- 1 tennaB, which acts as a reflector, this system being unidirectional at two frequencies, F and f. The'diiference D between the electrical lengths of the paths from the common energy supply point E of the radiating and reflecting antennae which are to be corrected and the free ends of the respective antennae, is made equal to an odd number of quarter wave lengths at one of the two frequencies, for example, frequency F. In the simple cases illustrated in this figure, as well as the other figures of the drawing, the transmission lines extend rectangularly and so this difference D is also the distance between the radiator and the reflector. However, the antennae or the distances from the common energy supply point E to the antennae, or both of these dimensions, may be of dissimilar length within the scope of my invention. At the second frequency this distance is not necessarily equal to an odd number of quarter wave lengths. The cancellation of back radiation at the two frequencies' is achieved in the following manner. The junctions 2% and 289 are located approximately one-quarter wave length away from the ends of the radiators and at voltage minima at frequency F. For this reason, at this frequency the auxiliary wires EEG-2M, 2092l0 produce substantially no effect on the operation of radiator A, so that the antenna operates as a unidirectional antenna in the well known manner. At the second frequency f, the voltage nodes are no longer located at junctions 2% and 209 but are displaced along the antenna either toward" the ends or away from them, so that the auxiliary wires 205-201,. mam produce a substantial effect on the distribution of currents along the antenna. In accordance with the present invention it is this change incurrent which is utilized to make the antenna system unidirectional at .the second frequency.

The radiation from a long wire antenna such as A may be divided into two parts, the radiation coming from the traveling waves traveling toward the ends of the antenna conductors, that is the direct waves, and the radiation from traveling waves traveling away from the ends of the antenna conductors, that is the reflected waves. The direct waves produceradiation mainly in the forward direction while the reflectedwaves produce radiation mainly in the backward di rection. The radiations produced by the direct and the reflected traveling waves are substantially independent of each other. For this reason, if the phase of the reflected traveling waves were changed at the end of the wire, for example, by changing the phase of reflection, then the phase ofthe back radiation would also be changed without substantially altering the phase or the magnitude of the forward radiation. The two radiations, the forward radiation and the backward radiation, would still be substantiallyindependent of each other. i

In accordance with my invention in the antenna system shown in Fig. 1, the lengths of auxiliary wires 2B52 ll1, 2092 iii are adjusted in such manner that the reflected waves of frequency 1 along the antenna conductors start out in such phase that the radiation produced by them cancels the radiation produced by the reflected waves traveling along the conductors of the. reflector. The phase of the :forward wave traveling along a conductor of the forward radiator at some point 25! differs from the phase of the forward'wave traveling along the conductor of the reflector at the corresponding point 250 in this conductor by an amount equivalent to difference D between the electrical lengths of the reflector and the radiator. Thus, if the forward waves were allowed to proceed to the ends of two antennae they would reradiator is either advanced or delayed as ,de-

sired, by means of auxiliary wires ZINE-Elli, 2G-2ill in order that thereflected waves on the radiator may be delayed or advanced by an amount M such that (2 D+lVl is equal to an odd number of one-half wave lengths. The following simple calculations enable one to arrive atv the approximate lengths of the auxiliary wires which are required to obtain the desired result.

If we neglect, for the moment, the total resistance'of wires 203-2), 209-2l i, and assume that the surge impedance of the antenna conductor as well as of the auxiliary wires is Z0 then the impedance of wire 289-21! is given by "7 Z1:- j cot l (l) and the impedance of wire 209-4"! is given by 2'. Z j cot -3 (2) Where L1 is the length of wire 209-2 and L2 is the length of wire 2li92l0. The impedance Z of both wires in parallel is then given by The reflection coeflicient' of this impedance when it is connected atthe end of an antenna wire of surge impedance Z0 is given by v I Z0 Z where p is the absolute value of the reflection coeflicient and 0c is the angle by which the phase of the reflected wave is advanced ahead of the forward wave which arrives at terminal 269 where impedance Z' is connected. Since we have assumed that the resistance of the wires may be disregarded the absolute Value of the reflection coeflicient is equal to unity. If we put The following example illustrateshow theabove equations may be used to calculate the dimensions of the auxiliary wires. Suppose that the first frequency at which the antenna is to operate is F=l5 megacycles While the second frequency is f=l0 megacycles and that the distance betweenv the. antennae D equal 1 wave lengths at 10 megacycles. Then at 15 megacycles this distance is equal to 1 .875 wave lengths, Accordingly, if

there were no auxiliary wires the back radiation from A would be delayed by 3.75 wave lengths in reference to radiation from B and would fail to cancel radiation B by .25 wave length. Under these circumstances the antenna system would be really unidirectional. The auxiliary wires 20E2ll'i, 2&9-2 Ill in accordance with the present invention should; therefore, be chosen so as to advance the phase of" the reflected wave by .25 wave length. Now if the length L2 of wire 2il92iil were 0, that is, if there is no auxiliary wire at all, then at this frequency the reflectioncoefficient of wire 2092ll at point 289 is given by Equation 7 in which L2 is equal to 0. From this equation it follows that an A Inorder to advance the reflected wave by the required amount of .25 of a wave length or 90 it is necessary that the auxiliary wire 209--2l0 be of such length that 0: is equal to 90 plus degrees. Substituting this new value of a into Equation 7 as well as the value of L1 we find that L2=.25 A at 15 megacycles.

In actual practice it will be found that this value is not entirely correct since we have not considered either the effect of radiation resistance or the effect of inter-action between the wire 209-2! and wires 2092H and 209205. Because of these sources of error in the above simple formulae it is best to calculate the desired length of 2U9-2| 0 approximately by means of Equation '7 and then to accurately adjust it by experiment. This experimental adjustment may be carried out as follows: The minima and the maxima of the standing wave pattern formed by interference of the forward and reflected waves may be readily found by means of a slide wire meter well known in the art. When the reflected wave is advanced in phase by say 90 the standing wave pattern will moveaway from the point of reflection by A of this amount, namely For this reason all that is necessary for adjustment is to ascertain the position of say the current minimum on the antenna wire, then to attach the auxiliary wire and to vary its length until the minimum has moved the required amount and in the required direction from the .old position.

It will be found that the cauculated value of auxiliary wire will generally speaking result in an approximately correct displacement of the standing wave pattern. However, experimental adjustment is generally desirable in order to ensure greater accuracy. I have found that it is entirely practical to make the required adjustments quite close to ground. If wire 205-2H is located 15 feet or so above ground so that it can be reached from a step ladder and the auxiliary wire 2fl92l| is pulled at some angle to the ground so that the end of it is 6 or '7 feet above ground the displacement of the standing wave pattern will be very nearly the same as if all of the said wires were pulled up to their normal place or feet above ground. The correction which must be made for the height is generally very small amounting to only several inches at frequencies of the order of 10 to 15 megacycles. This correction may be determined by pulling up the wires and rechecking the position of the standing wave pattern.

In choosing the position of junction 209 where the auxiliary wire is attached, the so-called end effect should be taken into account, that is, the length 209-211 should not be equal to exactly of a wave length but should be made somewhat shorter than a A; wave length. The exact length may best be determined by experiment since the end effect depends upon the type of insulator which is used at the end of the antenna wire. When No. 6 A. W. G. gauge wire is employed together with the usual type of insulators with two inch corona shields the end effect amounts to about 2 or 2%; feet at frequencies of the order of from 10 to 15 megacycles and does not vary a great deal with frequency. In any case, however, the auxiliary wire 209-4 I ll should be attached at voltage minimum which is also, of course, current maximum and which may be readily ascertained by means of a high frequency voltmeter or by means of a slidemeter.

If it is assumed that the radiation resistances of wires 209-210, 2092ll are equal to zero it is found that the absolute value of the reflection coefiicient p in Equation 4 is equal to unity. In actuality, of course, that is not correct so that the amplitude of the reflected wave is not equal to the amplitude of the forward wave which arrives at junction 209. For this reason if the antennae A and B are of equal dimensions the reflected wave which travels along radiator A is somewhat smaller in amplitude than the reflected wave which travels along radiator B and consequently the radiation fields which are produced by these two reflected waves even if out of phase do not completely cancel. This discrepancy may be reduced or entirely eliminated either by making the antennae of different lengths or of different radiating efficiencies or by deliberately supplying more power to the forward radiator. Another and preferred arrangement which does not require dissimilarity of antennae characteristics is shown in Fig. 2.

In this figure both the radiator and the reflector are equipped with auxiliary wires. The auxiliary wires 306-301, 309-4"! on radiator A are employed to advance the phase of the reflected wave at a given frequency while the corresponding auxiliary wires 3|23l3, 3I5--3l6 on radiator B are employed to delay the phase of the reflected wave at the same frequency, or vice versa. For example, if it is required to advance the reflected wave on radiator A by 90 in reference to the reflected wave on radiator B, only half of the total required advance is taken care of by auxiliary wires on radiator A and the other half of the relative advance is obtained by delaying the reflected wave on radiator B. Consequently this time the wires 306-301, 309-3!!! connected to radiator A are advancing the phase only 45 while wires 3l2-3l3, 3|53l6 attached to radiator B are delaying the phase by 45. When four auxiliary wires are employed in accordance with Fig. 2 the reflected waves which result on radiator B are of approximately the same amplitude as the reflected Waves which result on radiator A so that a more complete cancellation of back signal is secured.

The systems shown in Fig. 2 and Fig. 1 have been found quite satisfactory in actual practice when the ratio of the two frequencies is of the order of 1.2 to 1.5. When, however, the ratio of the two frequencies is of the order of 1.05, the circulating currents which are produced in the auxiliary wires become fairly large and the amount of side radiation which comes from these auxiliary wires also increases to what may be an undesirable extent.

It is possible to overcome these difficulties without undesirably lengthening the conductor between the point of attachment of the auxiliary wires and the'end of the antenna in terms of quarter wavelengths, by inserting in the said conductor an inductance coil, which, together with the remaining straight portion of the said conductor constitute a nearly pure reactive impedance which has a relatively great difference in value for a relatively small difference in the applied frequency. The same remarks also apply to the auxiliary wire which may have inserted therein an inductance coil, the remaining end of the said wire constituting a capacity.

A structure of this type is shown in Fig. 3. In

iii;-

this figure in place of the auxiliary wires shown 76 in-'Fig'. 2., elements 4ll9-42f, 4204i0 are employed. Eachelement consists of a coil l2l420 and two lengths of wire ate -s25 and 42G4l3. With an element of this kind a wide range of nearly'lpure reactive'impedances may be secured. The short length of wire 42fl4ll. acts as a capacitive impedance when seenfrom terminal 420. The coil, of course, has an inductive impedance which tends to partially balance out the capacitive impedance of wire 42fi5l0. The total impedance which is obtained at terminal 42f looking into the coil is the algebraic sum of the high negative reactance of wire 429-4.! and the positive reactance of coil 42ll29. It has been found convenient in practice to use coils which have impedances of the order of 600 ohms. When using such coils it is found that the tail Mil-4H3 at frequencies of the order of 15 megacycles has a rather short length, namely, of the order of 3 to 6 feet. In order to cut down mutual reactance between coils 421-4213, 423-422, it is desirable to employ lengths of wire such as 4 69- tZl between the coils and the junction point 489. If the coils have a diameter of 6 inches or so these lengths of wire may be a foot or a foot In. l

and a half long and have about 8 to 12 turns. order that coils may be easily replaced in they are damaged by falling on the ground or in some other manner it is best to make all coils exactly alike electrically so that they would beentirely interchangeable and so that a spare coil may be held in store ready for any emergency that might arise. all of the adjustments may be conveniently made by varying the lengths of the auxiliary wires of 428Mt, 323- 1! 1, etc. The lengths of the auxiliary wires may be adjusted in the follow ing manner. First, the lengths of auxiliary wires such as 423 ll I, are adjusted for minimum current in coils such as 42 l-AZD at the frequency at which the distance between the antennae is equal to an odd number of quarter waves. Second the lengths of wires such as 42li4lfl are adjusted until the required amounts of delay or advance are obtained at the second frequency. The latter adjustment is quite similar to the adjustments of auxiliary wires which were already fully described heretofore in connection with Fig. 2.

In Fig. 4 is shown still another modification of the present invention. In this figure there is shown anantenna system consisting of two antennae which are spaced a distance 5B4 5G5=D from each other which is not equal to an odd number of quarter wave lengths at either of the frequencies f and F at which this antenna system operates.

The unidirectionality at both frequencies in accordance with the present invention is secured by means of auxiliary wires in the following manner. At frequency f the required amount of delay or advance of the reflected waves is obtained by means of wires SUB-5M, EBB-5H1 which are attached at a voltage minimum at frequency F located approximately a quarter wavelength of frequency F from the ends 5B85i l of the antenna wires. Unidirectionality at frequency F is secured by means of auxiliary wires 5l25l3, 5l5--l6, which are attached at a voltage minimum at. frequency 1 located approximately a quarter wavelength of frequency 1 from ends El i-5|! of the antenna wires.

When it is desired to secure a more complete cancellation ofback signals the antennae may be made of unequal lengths or the total required advance may be divided into two preferably equal This is entirely practical as parts and be secured by means of four auxiliary wires, two connected to the radiator and two connected to the reflector.

In the preceding portions of the specification and in the formulae the assumption has been made that the waves to be canceled are radiated backward in line with the antennae rather than at an angle thereto. This assumption is satisfactory for small angles of radiation where the cosine is nearly unity. But in cases where the angle of radiation isrelatively large so that the cosine is no longer substantially unity, or where more complete cancellation of waves radiated at an angle is desired, a correction of the calculations must be made, in a manner obvious to one skilled in the art, to assure that the phase adjustment between the two waves is such as to assure the desired cancellation.

In general the backward wave of the forward antenna or radiator will have to be delayed more for complete cancellation, the greater the angle at which it is radiated.

While I have referred particularly to the use of antennae systems embodying my invention for transmission purposes, they may be used, also for the directional reception of signals. It will be obvious to one skilled in the art that the particular embodiments of my invention described above are for the purposes of illustration and that various modifications and adaptations. thereof may be made within the spirit of the invention as set forth in the appended claims.

I claim:

1. A directive antenna system comprising a radiating antenna and a reflecting antenna spaced apart a distance such that the back radiations from said antenna do not normally cancel, each antenna comprising an antenna conductor, an auxiliary wire connected branchingly toeach antenna conductor near the end thereof to ad- -just the phase of the back radiations to bring said back radiations into phase opposition and thereby reduce the effective strength thereof and an inductance coil serially connected in each antenna conductor still nearer thereof.

2; Adirective antenna system comprising a radiating antenna and a reflecting antenna for transmitting in a particular forward direction, said antennae being spaced apart a distance other than an odd number of quarter wavelengths of the wave to be radiated and said antennae having the same physical length but having auxiliary means for making the electrical lengths thereof of such difference that the back radiation from one antenna is in phase opposition to the back radiation from the other antenna.

3. A directive antenna system for the unidirectional transmission of a plurality of different frequencies comprising a radiating antenna and a reflecting antenna of equal physical length, said antennae being spaced apart a distance an odd number of quarter wavelengths of one of said frequencies and other than an odd number of quarter wavelengths at another of said frequencies, and auxiliary means ineffective at said one of said frequencies for making the electrical length of said antennae at said other frequency such that the back radiation from one antenna is in phase opposition to the back radiation from the other antenna at said other frequency.

4. A directive antenna system for the unidirectional transmission of different frequencies comprising a radiating antenna and a reflecting to the end antenna spaced apart a distance such that the 75 back radiations of one frequency do not normally cancel, while the back radiations of another frequency normally cancel, and phase delay means cooperating with at least one of said antennae to adjust the phase of at least one of said back radiations to said one frequency only to bring said back radiations of said one frequency into phase opposition and reduce the total effect thereof.

5. A directive antenna system for the unidirectional transmission of two different frequencies comprising a radiating antenna and a reflecting antenna spaced apart a distance such that the back radiations of one frequency do not normally cancel, while the back radiations of the other frequency normally cancel, each antenna comprising an antenna conductor and an impedance branchingly connected to the antenna conductor of one antenna near the end thereof at'a voltage minimum for said other frequency to adjust the phase of the back radiations of said one frequency to bring said back radiations into phase opposition and thereby reduce the effective strength thereof.

6. A directive antenna system for the unidirectional transmission of two different frequencies comprising a radiating antenna and a reflecting antenna spaced apart a distance such that the back radiations of one frequency do not normally cancel, while the back radiations of the other frequency normally cancel, said antenna comprising an antenna conductor and an impedance branchingly connected to each antenna conductor near the end thereof at a voltage minimum for said other frequency to adjust the phase of the back radiations of said one frequency to bring said back radiations into phase opposition and thereby reduce the effective strength thereof.

7. A directive antenna system for the unidirectional transmission of two frequencies which differ only slightly, comprising a radiating antenna and a reflecting antenna spaced apart a distance such that the back radiations of one frequency do not normally cancel, while the back radiations of the other frequency normally cancel, said antenna comprising an antenna conductor and an inductive impedance connected in series with said antenna conductor near the end thereof and a second impedance connected branchingly to said antenna conductor near the end thereof but between said impedance first mentioned and the point of energy supply to said antenna, the point of connection of said second impedance being at a Voltage minimum for said other frequency, and said impedances being of such value as to adjust the phase of the back radiations of said one frequency to bring said back radiations into phase opposition and thereby reduce the effective strength thereof.

8. A directive antenna system for the unidirectional transmission of two frequencies which differ only slightly, comprising a radiating antenna and a reflecting antenna spaced apart a distance such that the back radiations of one frequency do not normally cancel, while the back radiations of the other frequency normally cancel, each said antenna comprising an antenna conductor and an inductive impedance connected in series with eachof said antennae conductors near the end thereof and a second impedance connected branchingly to each of said antennae conductors near the end thereof but between said impedance'first mentioned and the point of energy supply to said antenna, the point of connection of said second impedance being at a voltage minimum for said other frequency and said impedances' being of-such value as to adjust the phase of the back radiations of said one frequency to bring said back radiations into phase opposition and thereby reduce the effective strength thereof.

9. An antenna system for transmitting a wave in a given direction, comprising a radiating antenna and a reflecting antenna spaced apart a distance such that the back radiations from said antennae do not normally cancel, each said antenna comprising an antenna conductor and impedance means connected branchingly to at least one of said antennae conductors intermediate the ends thereof to adjust the phase of only the back radiations of said one of said antennae to bring said back radiations from said antennae into phase opposition and thereby reduce the effective strength thereof.

10. A directive antenna system comprising a radiating antenna and a reflecting antenna spaced apart a distance such that the back radiations from said antennae do not normally can cel, each said antenna comprising an antenna conductor and an auxiliary wire impedance means connected branchingly to at least one of said antennae conductors intermediate the ends thereof to adjust the phase of at least one of said back radiations to bring said back radiations from said antennae into phase opposition and thereby reduce the effective strength thereof.

11. A directive antenna system comprising a radiating antennaand a reflecting antenna spaced apart a distance such that the back radiations from said antennae do not normally cancel each said antenna comprising an antenna conductor and an impedance connected branchingly to at least one of said antennae conductors at an intermediate point thereof near the end thereof to adjust the phase of at least one of said back radiations to bring said back radiations from said antennae into phase opposition and thereby reduce the effective strength thereof.

12. A directive antenna system comprising a radiating antenna and a reflecting antenna spaced apart a distance such that the back radiations from said antennae do not normally cancel, each said antenna comprising an antenna conductor and an impedance connected branchingly to each antenna at an intermediate point near the end thereof to adjust the phase of the back radiations to bring said back radiations into phase opposition and thereby reduce the effective strength thereof.

13. A directive antenna system comprising a radiating antenna and a reflecting antenna spaced apart a distance such that the back radiations from said antenna do not normally cancel, each said antenna comprising an antenna conductor and an auxiliary wire connected branchingly to each antenna conductor at an intermediate point near the end thereof to adjust the phase of said back radiations to bring said back radiations into phase opposition and thereby reduce the effective strength thereof.

14. A directive antenna system for the unidirectional transmission of two frequencies which differ only slightly, comprising a radiating antenna and a reflecting antenna spaced apart a distance such that the back radiations of one frequency do not normally cancel, while the back radiations of the other frequency normally cancel, each said antenna comprising an antenna conductor and. an impedance connected branchingly to one antenna conductor near the end thereof at a point of voltage minimum at said other frequency, said impedance being of such value that the back radiations of both frequencies are brought into phase opposition and the. effective strength thereof reduced.

15. An antenna system for transmitting av wave in a given direction comprising a radiating antenna and a reflecting antenna spacedapart a distance such that the back radiations from said antennae do not normally cancel, each said antenna comprising an antenna conductor and impedance means comprising an inductance coil connected branchingly to at least one of said antennae conductors toadjust the phase of only the back radiations of said one of said antennae ductor and an impedance comprising an inductance coil connected branchingly to each antenna conductor near the end thereof to adjust the phase of the back radiations to bring said back radiations into phase opposition and thereby reduce the effective strength thereof.

ANDREW ALFORD. 

